A so-called sandwich panel has been conventionally used for many purposes such as transport machineries, e.g., automobiles or aircrafts, building materials, electrical device housings, sports/leisure, and others.
The sandwich panel has two skin material sheets and a core interposed between both the skin material sheets, and a laminated structure including the skin material sheet, the core, and the skin material sheet is a basic conformation, but the function required for the sandwich panel varies depending on each intended purpose.
For example, dressed lumber is further bonded to a front surface-side skin material sheet that provides the external appearance when a high value is attached to attractive appearance but very high strength is not required like an interior panel used for a bathroom, and the higher strength rather than attractive appearance is required when a high value is attached to a surface texture of the dressed lumber or an entire formed shape and an intended purpose is a building material.
In this regard, since a reduction of weight is required and strength is also required for interior material components for transportation machinery such as automobiles or aircrafts, especially a cargo floor board for a vehicle, a deck board, a rear parcel shelf, and others in light of improvement of fuel efficiency, a resin sandwich panel in which both skin material sheets and a core are made of a resin has been often used. In conventional examples, to cope with an outer shape for an intended use, such a sandwich panel is brought to completion by manufacturing both the skin material sheets and the core by, e.g., a cut process and assembling and bonding both the prepared skin material sheets and core.
On the other hand, the resin sandwich panel having both the skin material sheets and the core made of a resin is manufactured by various molding methods.
Patent Document 1 discloses a method of forming a resin sandwich panel by extrusion molding.
According to this molding method, each surface layer constituting the sandwich panel is extruded from a T-die, and each surface layer is welded by using melting heat of the molten surface layer, whereby the sandwich panel having excellent interlayer adhesive properties can be continuously manufactured.
Patent Document 2 discloses a method of forming a resin sandwich panel by injection molding.
According to this molding method, a necessary amount of a composition forming a skin layer in a sandwich panel having the skin layer and an internal layer made of different compositions, which is required for forming a thickness of the upholstery layer, is injected from a first cylinder, and then a composition forming the internal layer is injected from a second cylinder at a high rate, thereby molding the sandwich panel having the thin skin layer.
According to the method of forming the resin sandwich panel based on such injection molding, it is possible to manufacture not only a molded piece having a fixed cross-sectional shape but also a molded piece having a free outer shape whose cross-sectional shape changes, and restrictions on the shape of a molded piece are moderate as compared with the extrusion molding in this regard.
However, since the injection molding method adopts a conformation that a molten resin is formed by injecting the molten resin into a closed mold space and pressing the molten resin against a closed mold inner surface, molding a molded piece having a closed hollow portion therein by the injection molding method alone is technically difficult.
Meanwhile, for example, when utilizing the resin sandwich panel as a cargo floor lid of an automobile, since the resin sandwich panel is used for not only an attractive appearance but also an application of mounting heavy cargos on the cargo floor lid, rigidity for weight-bearing of the cargos (flexural rigidity in particular) is required, whereas a reduction in weight is demanded in terms of improvement of fuel efficiency, and a technical problem that achieving both high rigidity and reduction in weight is difficult, must be overcome.
Therefore, as the resin sandwich panel for such an intended use, a hard resin material having a high Young's modulus is adopted for each skin material sheet and, on the other hand, a dimension (a thickness of a core) of the core is increased and a gap between both the skin material sheets is enlarged as much as possible to raise a section modulus, and a foam material is adopted, a hollow portion is provided inside, or a piece having many recesses provided on a surface is adopted for the core itself in order to reduce its weight, for example.
In more detail, in the resin sandwich panel, provision of voids in the core to achieve a reduction in weight is roughly divided into two methods, i.e., foaming a resin to provide a limitless number of air bubbles inside as disclosed in, e.g., Patent Document 3 to Patent Document 7 and providing many concave portions on the surface of the resin material as disclosed in, e.g., Patent Document 8.
As the conformation that many concave portions are provided as disclosed in Patent Document 8, two resin sheets are used, each sheet has on the outer surface thereof a plurality of recesses constituted of annular ribs protruding toward each inner surface side, each of the plurality of recesses have a butt planar portion on the bottom, and a hollow portion is formed between the two resin sheets except a butt-welded portion by performing butt-welding in such a manner that the planar portions of the corresponding recesses on the respective two resin sheets are welded back to back.
Each of Patent Document 3 to Patent Document 7 discloses a sandwich structure constituted of at least a three-layer structure in which a foamed core is sandwiched between a pair of upper and lower skin materials.
Patent Document 3 discloses a point that carbon fiber reinforced sheets are bonded to both surfaces of resin foam consisting of a polyetherimide resin having an expansion ratio of approximately 15- to 30-fold by vacuum suction as an airframe structure of an aircraft. Patent Document 4 to Patent Document 7 disclose a common sandwich structure for an interior material panel such as a deck board or a floor panel of an automobile. Patent Document 4 discloses a point that two molten parisons are positioned between split molds, a previously shaped foamed core such as polypropylene is arranged between the two molten parisons, and the foamed core is pressed against the split molds to be molded in a vacuum or pneumatic state. Patent Document 5 discloses a point that a softened material obtained by reheating a previously shaped back surface-side sheet material is mounted in a lower one of upper and lower split molds, then the back surface-side sheet material is vacuum-sucked to be formed through the lower mold, resin foam such as polyethylene is mounted on the back surface-side sheet material, and a softened material obtained by reheating a previously shaped front surface-side sheet material is mounted on the resin foam, and mold clamping is subsequently carried out while performing vacuum suction through the upper and lower molds, thereby molding a sandwich structure. Patent Document 5 further discloses a point that a molded piece having a lattice structure, a molded piece having a honeycomb structure, or a molded piece having an embossed structure previously formed by punching processing or injection molding is used in place of such a resin foam core and a sandwich structure is molded by the same manufacturing method.
Patent Document 6 discloses a point that a laminated structure is manufactured by integral press working using foamed beads of an acrylonitrile-styrene copolymer having an expansion ratio of four- to 12-fold as foam.
Patent Document 7 discloses a point that a multilayer panel is manufactured by press working using foamed beads of polystyrene as the foam like Patent Document 6.
On the other hand, Patent Document 8 discloses a sandwich panel having a resin core that has many recesses each having a circular truncated cone shape, the sandwich panel being molded based on extrusion molding using corrugated rolls. This sandwich panel has two resin skin material sheets and a thermoplastic resin core interposed between both the skin material sheets, the resin core has a pair of thermoplastic resin plates each having a plurality of circular truncated conical recesses tapered toward the inner side on the outer surface thereof. Each of the plurality of circular truncated conical recesses is formed by a plurality of protrusions provided on the surface of each corrugated roll, protrudes toward the inner surface side, and has a butt planar portion at the most tapered part. The core having a hollow rib structure can be formed by butt-welding the planar portions of the corresponding recesses on the pair of resin plates based on pressing force of the corrugated rolls.
According to the above-described sandwich panel, although a reduction in weight can be achieved by forming the air bubbles based on foaming or forming the recesses on the surfaces, the following technical problems remain.
At first, when a reduction in weight is attempted by the formation of air bubbles or the formation of recesses on the surfaces alone, other functions such as strength, heat insulating properties, sound absorbency, and the like are deteriorated.
In more detail, with regard to Patent Document 3 to Patent Document 7, for example, a reduction in weight can be readily achieved by simply increasing a fill of a foaming agent to raise the expansion ratio and, on the other hand, an adhesion area between the core and the skin material is not reduced since openings do not have to be formed in the surface of the core, whereas compression rigidity of the core itself is lowered because an unlimited number of air bubbles uniformly spread in the entire core.
Further, in the case of Patent Document 3 to Patent Document 7, although the heat insulating properties or the sound absorbency can be improved in the entire core by raising the expansion ratio, a desired rigidity distribution is hardly obtained since adjusting air bubble forming positions is difficult.
In particular, since a closed cell percentage tends to lower by raising the expansion ratio, the heat insulating properties are thus considerably decreased.
On the other hand, in case of Patent Document 8, a reduction in weight can be achieved by providing the concave portions and, on the other hand, the compression rigidity of the core itself can be assured by the annular ribs constituting each concave portion, whereas openings must be necessarily provided in the surface of the core. Therefore, an adhesion area between the core and the skin material is reduced, and the internal configuration of the core is complicated by increasing the number of the concave portions, whereby a special molding method will be required.
If the bonding area between the core and the skin material is reduced, the flexural rigidity or the shear rigidity of the entire sandwich panel is decreased. On the other hand, in a case where a bending load is imposed on the sandwich panel, since the maximum bending stress is produced in the skin material sheet at upper and lower ends, the core itself may be possibly destroyed or damaged even if a dimension of the core is assured and firm adhesive properties between the core and the skin sheet are also assured, and hence strength of the core itself must be assured.
Furthermore, in the case of Patent Document 8, although a desired distribution of the concave portions and a rigidity distribution can be obtained by adjusting a position, a shape, or a size of each protruding portion provided on a cavity of the mold, a void volume of each concave portion is unavoidably extremely larger than that of the air bubble obtained by foaming, and hence improving the heat insulating properties or the sound absorbency is difficult.
Second, obtaining the sandwich panel of good quality is difficult because of the method of forming a sandwich panel.
In more detail, like Patent Document 3 to Patent Document 7, although a chemical foaming technology or a physical foaming technology is generally used for forming air bubbles by foaming, foaming does not restrict the molding method for the core in both the technologies, and extrusion molding, injection molding, blow molding, or press molding can be utilized like an unfoamed core, but an increase in the expansion ratio creates difficulty in adjustment of the expansion ratio in case of the extrusion molding or the injection molding in particular, thereby making it difficult to obtain desired quality.
On the other hand, like Patent Document 8, when forming the recesses on the surface, adhesive properties of the pair of thermoplastic resin plates constituting the core are poor, and the sufficient flexural rigidity of the entire sandwich panel cannot be exercised.
In more detail, since the pair of resin plates are fed to a space between the corrugated rolls and pressing force is applied at this position to effect bonding, bonding of the pair of resin plates is close to point bonding rather than surface bonding, and a bonding time is short, whereby sufficient adhesive properties cannot be assured.
In this regard, according to molding using a so-called mold, a tapered angle, which is narrowed toward the inner side, must be provided to the plurality of recesses because of a restriction in the molding using the mold, but a sufficient welding time can be assured by mold clamping, and thus, such a problem does not possibly occur.
Patent Document 9 discloses a method for manufacturing a resin sandwich panel based on sheet molding (compression molding) which is one type of molding using a mold.
In more detail, skin material sheets and a core (dressed lumber according to circumstances) are arranged between two split molds, the skin material sheets and the core in the molds can be pressurized and formed by clamping the split molds, and the skin material sheets and the core can be welded, whereby the sandwich panel can be formed at a time in this regard.
However, to effect forming and welding by mold clamping, both the skin material sheets and the core must be molten. If the skin material sheets are turned out as a continuous sheet from, e.g., an original fabric roll, they must be reheated by, e.g., an infrared heater before being arranged in the split molds. When using the sandwich panel for an application as a structure due to such reheating, obtaining the sandwich panel with sufficient strength is difficult. In more detail, the welding strength of an outer peripheral parting line which is a welding part of the skin material sheets or welding strength between the skin material and the core is deteriorated due to an adverse effect of the above-described reheating on moldability, thereby lowering strength of the entire sandwich panel.